Chips from whole trees or similar material provides an inexpensive source of wood and many pulp mills are now using varying proportions of these chips together with the regular chips. However, chips from whole trees generally contain a significant amount of long twigs or branch stems that cause costly handling problems, for example, hung-ups in chip bins are metering valves etc. and use of these chips is restricted. The industry in the near future may be forced by government regulations to further utilization of the cut tree and whole tree chipping may be more widespread.
Convential disc chippers including those used to chip whole trees include a housing with a disc rotably mounted therein and provided with slots therethrough and knives. The cut chips pass through the slots into a chamber and are ejected from the chamber by suitable vanes projecting from the rear face of the disc. Twigs or branch stems are not easily cut by the knives and simply stay on the front face of the disc or sometimes progress to the back face of the disc either by passing through the chip slot or around the periphery of the disc and are thrown out the chipper spout mixed with the cut chips. No simple and efficient way of separating these twigs and branch stems and generally debris associated with chipping whole trees has been devised to date.
It has been proposed to divide the chipper housing into two compartments one of which is an involute and forms a fan to provide more air for conveying the chips as taught, for example, in Canadian Pat. No. 754,372 issued Mar. 14, 1967 to Eklund, however, this device does not separate debris from chips.
It has also been proposed to separate dirt and loose bark in a drum chipper designed to chip short length large diameter logs. In this chipper the short log lengths are fed radially (the longitudinal axis of the log and drum are aligned) and the knives cut substantially parallel to the grain to form chips which pass into the drum. This device is unsuitable for whole tree chipping.